Raman probe with spatial filter and semi-confocal lens

ABSTRACT

A material identification apparatus  10  uses a spectrograph and detector array detecting a Raman spectrum produced by a sample illuminated by a laser source to recognize a variety of materials. The Raman spectra are produced by materials illuminated by an inexpensive near-infrared multimode laser operated in a pulse mode to deliver between 0.05 and 0.5 joules of photon energy, with the Raman spectra being detected before any significant heating of the sample occurs. The identification apparatus  10  qualitatively determines the chemical composition of reinforced and unreinforced copolymers and composites such as ABS, polypropylene, talc-filled polypropylene, polycarbonate, PMMA, PVB, polyethylene, and PVC, from samples of different colors, layers, and textures with a high degree of success without the need for special sample preparation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of PCT/US98/13856, filed Jul.2, 1998, which is in turn based on United States Provisional ApplicationSerial No. 60/051,533 filed Jul. 2, 1997, which are hereby incorporatedherein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to applied optics,spectra-chemical image processing, chemical identification, and chemicalanalysis. More specifically, the present invention relates to apparatusand methods for the non-destructive testing and identification of thecomposition of a sample of plastics or other materials using Ramanspectroscopy and computerized signal processing. The present inventionparticularly relates to apparatus for identifying the composition ofvarious layers of a material beyond merely the surface layer.

A widespread need exists in industry and government for identifyingmaterials. For example, automobile companies and plastics manufacturesmust identify and separate plastic resins in recycling operations.Pharmaceutical companies must monitor chemical constituents during drugproduction. Monitoring agencies and firms must assay waste stream flowsinto the environment. Law enforcement agencies must identify thepresence of illicit drugs in the field in order to combat criminal drugtrafficking.

In today's environmentally conscious society, simple economics providesa strong incentive for manufacturers to minimize their use of naturalresources and public landfills. Substantial economic benefit can begained by turning to recycling as a source for raw materials and as theultimate repository for the manufactured goods. Using recycled rawmaterials in products can increase profits by saving materials costs andenergy. Efficiently recycling packaging and production waste can savelandfill charges and provide a cost recovery stream. Further,manufacturing goods with recycled content, and designing goods thatthemselves are recyclable, is a civic duty that also offers publicrelations benefits that are worthwhile from a marketing standpoint.

Many suppliers, however, face difficulties in using recycled feedstreams.

All companies face competition, and in the marketplace, price alone doesnot guarantee market share. Most manufacturers value quality andconsistency of goods more than the abstract notions of civic duty andenvironmental policy.

Further, many production lines operate with just-in-time inventories, inwhich a factory receives all the components necessary to assemble aproduct only hours before they are needed. The presence of only a fewdefective parts can shut down a production line until replacement partsarrive. Such a shut down can cost a manufacturer many thousands ofdollars.

Thus, it is easy to understand why companies have been reluctant toinclude recycled materials in products. Recycled materials must havedocumented histories so that they are assured of compatibility with themanufacturing process. A misidentified piece of recycled materialincluded with virgin material can destroy an entire production run.Maintaining the history of recycled goods, or even knowing their exactcomposition is difficult, if not impossible, with current technology.

Millions of tons of plastic and other materials are deposited inlandfills or incinerated every year due almost solely to the lack ofsufficient technology to avoid cross contamination between differenttypes of plastic or other material during collection. The need thereforeexists for an effective, economical means to identify a variety ofmaterials, and specifically plastics, on site in scrap yards,warehouses, factories and recycling centers. The successfulcommercialization of an instrument with such capabilities would greatlyincrease the recycling rates for plastics and perhaps many othermaterials. By offering a simple means to overcome the difficult problemof material identification, the present invention seeks to help makemanufacturers more receptive to including recycled content in theirproducts, and purchasers more confident of the quality of thoseproducts.

Many methods exist for identifying materials. One test for plasticmaterials, for example, involves the burning of a small sample of theplastic material. Upon smelling the smoke, a trained technician canidentify several different classes of plastics with reasonable success.While this method can be employed in a laboratory, such methods are notappropriate or practical for commercial or production line applications.This type of chemical analysis would also not be acceptable to lawenforcement personnel or the courts for the identification of Cocaine.

An assortment of analytical identification methods exist, such asFourier Transform Infrared Spectroscopy (FTIR) and X-ray fluorescence(XRF), for the non-destructive testing of materials. An example of FTIRtechnology is disclosed in U.S. Pat. Nos. 5,510,619. While well knownand used, the FTIR process is not practical in many commercialapplications because the method is very sensitive to dirt, surfaceroughness, coatings, moisture, and sample motion during identification.The XRF process is also used but it is relatively expensive. Otheranalytical identification methods are disclosed, for example, in U.S.Pat. Nos. 5,256,880 and 5,512,752.

Raman spectroscopy, discovered by C. D. Raman in 1928, has many uniquequalities that can be advantageously employed in the practicalidentification of materials. Raman signals, generated by the interactionof monochromatic light and a sample, are not affected by dirt, surfacefinish, coatings, or any motion of the sample being identified.Significantly, Raman signals are also not as sensitive to water, glassor quartz as other infrared signals. As a result, chemical samples canbe contained within a glass vessel, or even suspended in an aqueoussolution without affecting the Raman signal. The Raman process also hasa significantly higher working distance with a controllable depth offield than other processes and can “look through” a container to thechemical sample contained inside. In many instances, however, the Ramansignal from the glass is very strong as compared to the chemical samplecontained within the glass thus presenting a significant signal to noiseratio problem that must be solved in order for accurate identificationof the chemical sample contained within the glass container.

Despite these advantages, Raman spectroscopy is not widely used becauseof a low signal to noise ratio inherent in Raman Spectroscopy.Traditionally, the excitation light source, typically a laser, isdirected continuously against a chemical sample, and the Raman signal iscollected over time. An example of such an apparatus is disclosed inU.S. Pat. No. 5,534,997. Increasing the power of the excitation laser inthe Raman process can increase the strength of the Raman signal andreduce the required sampling time. However, the increase in power cancause thermal damage to the sample particularly if the sample has lowthermal conductivity that is typical of plastics. The increase in powercan also cause “black body” or thermal radiation that can overwhelm theRaman signal. It is commonly assumed, therefore, that Raman spectroscopyis not appropriate for the practical identification of highly energyabsorbent materials such as black or highly pigmented plastics. In theextreme case, such highly absorbent materials can char or burn thusrendering the material unsuitable for further use. As an alternative,optical concentrators such as are disclosed in U.S. Pat. No. 5,615,673can be employed to gather the Raman signal over a wider range of angles,but physical limitations suggest that the signal is unlikely to beenhanced by more than a factor of about 3, and any masking signal willbe enhanced by a similar factor leading to no improvement in signal tonoise ratio.

Another challenge is presented by plastics that are in situated inlayers and the identity of the plastic in question may be other than thefirst layer. In such circumstances it is desirable to recover the Ramansignal from only the layer of interest. For example, many plasticcontainers are today made of five or seven co-extruded layers of variousplastics that may be necessary to identify to determine initial productquality or whether recycling is possible. Another example is found inautomobile windshields and other “safety glass” in which two layers ofglass are bound together by a layer of plastic, such as a polyvinylbutyral resin (PVB), of various chemical compositional variation whichcan be important to the identification of source, quality, etc. In manyinstances, it is desirable to determine the composition of the insidelayers of such articles without physically directly accessing the layeror layers in question.

What is needed is a system for analyzing and identifying the compositionof a wide variety of materials that is fast enough for practicalapplication in a commercial setting, insensitive to sample impurities orsurface imperfections, tolerant of water and common sample containers,and which does not damage the sample being analyzed and identified.

SUMMARY OF THE INVENTION

A system satisfying these needs generally comprises a probe including ahousing having an optical window through which visible and infraredlight can pass. A monochromatic light source is provided withtransmission optics optically coupling the light source to the opticalwindow so that light emitted from the light source is directed throughthe window toward a sample causing the sample to produce acharacteristic Raman signal. Sampling optics are coupled to the opticalwindow to receive the characteristic Raman signal produced from thesample including a spatial filter for removing unwanted spectralportions. A spectrograph is coupled to the sampling optics by an opticalfiber bundle. The fiber bundle is coupled to an output of the spatialfilter and is arranged into a linear array at an input to thespectrograph that disperses the characteristic Raman signal into aspectrum to form a spectrographic output. An optical detector is coupledto the spectrograph to receive the spectrographic output and generate adigital map representing the Raman spectrum as a function of wavelength.A computer is coupled to the detector that includes an input to receivethe digital map, a library of digital maps of specimens of knowncomposition, a comparison means for comparing a received digital mapwith the contents of the library to select any match exceeding aspecified confidence level, and an output display of the name of theselected match. A trigger circuit is included that initiates collectionof the spectrographic output by the detector at the same time orslightly before it initiates emission by the light source so that dataconcerning the characteristic Raman signal from the sample can becollected by the detector before any significant heating of the samplecan take place. The trigger circuit initiates selectively one or morepulses of light from the light source of specified duration and power soas to avoid or reduce the black body or thermal radiation problemcommonly experienced with other apparatus. The apparatus is suitable foruse to identify a wide variety of materials in gas, liquid, solid, orpowder form, including polymers, plastics, ceramics, minerals,composites, pharmaceuticals, petrochemicals, organics, inorganics,biochemicals, and organo-metallics. To examine inside layers of materialfor composition, the sampling optics includes an objective lens systemhaving a very narrow depth of field, typically less than 1 mm, andpotentially as small as 10 μm, so that the system can selectively viewonly a desired layer of a sample.

The present invention advantageously employs one or more discrete pulsesof light, preferably generated by a laser, to cause the Raman signal tobe produced by a sample. The monochromatic light source preferablycomprises an inexpensive near-infrared multimode laser operable in apulse mode between a lower and an upper intensity. The lower intensitylevel is at or just above the threshold intensity below which laseractivity discontinues, typically less than about 105% of the lasingthreshold. The upper intensity can be at any level at or below a maximumintensity established by the upper power limit of the laser. A “lapsetime” or “off time” between the laser pulses is provided to allow thesample to dissipate localized heat generated by the laser pulse.

Advantageously, a single location on a sample can be exposed to morethan one pulse during the sampling process, and allowed to cool betweenpulses.

The laser pulses are cycled repeatedly until an adequate Raman signal iscollected to allow the required analysis to identify the sample. Thepulse width defining the duration of the upper intensity can be betweenabout 0.01 and 10.0 seconds, depending upon the nature of the sample. Inthe preferred embodiment employing a 1.2 Watt laser, the pulse durationis between about 80 and 120 milliseconds for materials sensitive tooptical damage. For less sensitive materials, a typical pulse durationcan be up to about 500 milliseconds. The pulse duration can be shortenedby using a correspondingly higher power laser to deliver between about0.05 to 2.0 joules of photon energy to the sample. The number andduration of pulses employed to test a given sample is controlled toprovide a satisfactory Raman signal while minimizing the thermal strainon the sample and avoiding detector saturation at any given pixelposition. Shorter pulses and multiple discrete pulses are an importantfeature of the present invention which advantageously provides anenhanced Raman signal to noise ratio, even when considering sources ofthe same power, as the thermal noise and likelihood of detectorsaturation is kept to a minimum. Pulse width, in the present invention,is defined as the full width, half maximum (FWHM) amplitude of a pulse.

In a preferred embodiment of the invention, the probe housing includes ahandle or other similar feature to permit manual or machine-poweredmanipulation of the probe, and further comprises a trigger that can besituated on the handle and coupled to the light source so thatdepression of the trigger temporarily increases the output of light fromthe source, preferably in the form of one or more prescribed pulses aspreviously described. The apparatus preferably includes a console forhousing the spectrograph, the optical detector, and the computer, andthe sampling optics include a fiber-optic umbilical coupled between theconsole and the probe. The light source is housed in the console and thetransmission optics includes at least one optical fiber carried by thefiber-optic umbilical. The probe housing preferably comprises anelongated rail having a lower and an upper surface, a longitudinal axisof the probe being separated by a fixed distance from the upper surface.A plurality of supports are fixed to the rail upper surface forsupporting optical elements to intersect the longitudinal axis of theprobe. The probe housing also includes a tubular member including alongitudinal slot that receives the rail so that an axis of revolutionof the tubular member is coincident with the longitudinal axis of theprobe and parallel to the rail upper surface. A back plate closes oneend of the tubular member while a nose cone containing the opticalwindow closes the other end. An off-axis baffling tube is supported bythe rail parallel to the longitudinal axis to define a specificsegregated region within the probe to permit extinction of unwantedportions of the light source spectrum immediately prior to illuminationof the sample. This design allows for an ease of construction thatcontributes to lowering the cost of the overall system sufficiently topermit widespread adoption of the system by both industry andgovernment.

In the preferred embodiment, the optical elements held by the pluralityof supports within the probe can include lenses providing the probe witha prescribed depth of field, at least one filter, and a reflectiveelement inclined with respect to the longitudinal axis of the probe foraligning the laser output with the optical window. To achieve optimumRaman signals, the lenses uniformly distribute the laser output over asample irradiation area so as to achieve a power density of betweenabout 0.1 W/mm² and 10 W/mm². This is achieved by employing lenseshaving an f-number of between about 1 and 4 The objective lens systemthat is situated adjacent to the sample to receive radiation scatteredfrom and emitted by the sample, including the characteristic Ramanspectrum produced by the sample, generates a collimated beam of theradiation. Preferably, the objective lens system has such a small depthof field as to operate essentially as a semi-confocal system. An opticalfilter is positioned to receive the collimated beam of radiation fromthe objective lens and allow past primarily only a selected spectralportion including the characteristic Raman spectrum produced from thesample. The optical filter can be a holographic notch filter or a longpass filter that selectively rejects light at the wavelength of thelaser source but transmits light of longer wavelength. A lens ispositioned to received the selected spectral portion passing through theoptical filter and direct it toward a focal point. A spatial filter ispositioned with respect to the lens so that an input is positioned atabout the focal point to receive the selected spectral portion. Thespatial filter attenuates unwanted radiation so that an output of thespatial filter receives substantially only the characteristic Ramanspectrum produced from the sample. The optical fiber is preferably abundle coupled to the output of the spatial filter for transmitting thecharacteristic Raman spectrum produced from the sample to thespectrometer apparatus for identifying the composition of the samplefrom the characteristic Raman spectra that was emitted from the sample.The entrance ends of the optical fibers forming the optical fiber bundleare situated in a compact array in a common plane within the probehousing so as maximally to collect the Raman signal produced by thesample. The exit ends of the optical fibers are arranged in a line at anentrance to the spectrograph that typically contains a diffractiongrating, the ruled surface of which is aligned to be parallel with theline defined by the exit ends of the optical fibers. This feature hasthe advantage of providing a signal to the spectrograph that is ofenhanced intensity over that which might be gathered by a single fiberyet without diminishing the spectrographic resolution. The opticaldetector comprises an array detector having a number of outputssufficient to define a division of the spectrographic output into a setof discrete spectral elements having a pixel width of about 0.2 nm.

A laser particularly suitable for use with the present invention is anear-infrared (NIR) multi-mode diode laser, the wavelength and deviationof which are compatible with spectrographic identification apparatus.The use of such a laser in the present invention provides an excitationsource of lower cost yet higher power and reliability than comparablesingle-mode diode laser. In the preferred embodiment, the laser is a 1.2Watt, 798±5 nm pulsed multi-mode laser excitation source having aspectral width of less than 2.5 nm and more typically about 1-2 nm. Theuse of an excitation source having a spectral width significantlygreater than the pixel width in the detector of the spectrographicoutput advantageously shifts the width of Raman peak features into aregime that is well separated from both pixel noise (of small width) andbackground emission (of larger width). This facilitates more efficientsmoothing (filtering) to reject noise and background with minimal lossin Raman spectral information. This smoothing function is particularlyimportant when derivatives of the spectrum are used for comparison asthe difference in wavelength permits a significant reduction in pixelnoise. The ratio of excitation source spectral width to spectraldetection element width is desirably between about 4 and 40, andpreferably between about 5 and 20. That is, the width of the spectrumdetected by each pixel of the detector is less than ¼^(th) andpreferably less than ⅕_(th) the excitation source wavelength deviation.

In a typical embodiment of the present invention, the computer can be ageneral-purpose personal computer programmed to contain a library ofdigital maps of various materials of known composition. In the preferredembodiment, the digital maps consist of a second-derivative vector ofthe spectrographic output as a function of Raman shift wavelengthsmoothed to exclude the short wavelength pixel noise. The resultingsecond derivative is treated as an n-dimensional vector where n is thenumber of pixels in the spectrographic output. The computer isprogrammed to compare the information taken from a sample of unknowncomposition with the library by computing a vector dot product of thevector of the sample with each specimen vector stored in the library.The dot product constitutes a scalar evaluation of coincidence betweeneach pair of vectors. Based on this computation, the computer can beprogrammed to select the best fit by merely retaining the identity ofthe known material having the highest value for the dot product, anddisplaying that identity. Where the library contains vectors of a numberof very closely related materials, the computer can be programmed toretain all library-stored vectors for which the vector dot product wasgreater than a specified value. The computer can then conduct a reviewof additional criteria of the sample vector that could discriminate theretained library vectors to select the best fit. This feature, whenemployed by even conventional personal computers having operating speedsin the order of 300 MHz, provides an accurate identification of a widevariety of materials, typically in only 1 or 2 seconds. Where very rapidsample identification in desired, for example in automated systems,dedicated digital signal processing equipment can be employed ratherthan general-purpose personal computers.

By offering a simple means to overcome the difficult problem ofidentification of materials for recycling, the present invention seeksto help make manufacturers more receptive to the idea of includingrecycled content in their products and give purchasers more confidencein the quality of those products. The same system programmed with aslightly different library of reference materials can enable companiesto monitor a wide variety chemical constituents during manufacture andpermit the assay waste stream flows into the environment for anyunwanted contamination. Law enforcement agencies are also able to usethe system to identify the presence of illicit drugs in the fieldwithout affecting the chemical make-up of the sample or detracting fromthe amount of material tested.

Additional features and advantages of the invention will become apparentto those skilled in the art upon consideration of the following detaileddescription of the preferred embodiment exemplifying the best mode ofcarrying out the invention as presently perceived.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an apparatus of the present inventionshowing an identification probe, a fiber-optic umbilical, and mobileconsole containing a laser, a spectrograph, a charge-coupled detector,and a data processing computer.

FIG. 2 is a perspective view of an identification probe of the presentinvention situated to test a sample which can be formed to include morethan one layer of material.

FIG. 3 is a sectional view taken along line 3—3 of FIG. 2.

FIG. 4 is an exploded perspective view of an identification probeincorporating a spatial filter of the present invention.

FIG. 5 is a horizontal sectional view of the identification probe shownin FIG. 4.

FIG. 6 is a horizontal detail view of the spatial filter employed inFIGS. 4 and 5.

FIG. 7 is a diagrammatic view of the principal elements included in anapparatus of the present invention.

FIG. 8 is a flow diagram of the identification method of the presentinvention.

FIG. 9 is a schematic view of the fiber bundle employed in the samplingoptics showing the preferred mapping of the optical fibers at the twoends of the fiber bundle.

FIGS. 10a-10 c are Raman spectra of a HDPE sample at various exposuretimes and sensitivities without use of a spatial filter of the presentinvention.

FIG. 10d is a Raman spectra of a HDPE sample using a spatial filter ofthe present invention.

FIGS. 11a-11 d are Raman spectra of other sample materials using aspatial filter of the present invention.

FIGS. 12a and 12 b are Raman spectra of a typical windshield glass andsafety layer of PVB-SR60 using a conventional objective lens system.

FIG. 12c is the second-derivative vectors of the two curves shown inFIG. 12a.

FIG. 13a shows the Raman spectra of two layers of a windshield using thesemi-confocal objective lens system of the present invention.

FIG. 13b shows the second-derivative vectors of the two curves shown inFIG. 13a.

FIG. 14 is a graph of Raman signal strength as a function of distance ofthe objective lens system from a PETE specimen.

FIG. 15 is an enlarged detail view of the graph of FIG. 14 for twosemi-confocal objective lens systems of the present invention.

FIG. 16 is a graph of depth of field (FWHM) as a function of focallength for various objective lens systems of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

An identification apparatus 10 of the present invention includes anidentification probe 12, which can be hand-held, attached by afiber-optic umbilical 14 to an instrument console 16 that houses alaser, a spectrometer, a charge-coupled array detector, and a computerhaving a display screen 18. In addition, instrument console 16 is fittedwith an external holder 17 to house probe 12 as shown in FIG. 1. Theinstrument console 16 preferably includes supporting wheels 15 so thatidentification apparatus 10 can easily be moved from one location to thenext. It is understood, however, that the instrument console 16 may bestationary without exceeding the scope of the present invention. Theinstrument console 16 preferably conforms to National ElectricalManufacturers Association-12 specifications so that console 16 protectsthe instruments contain therein from adverse environmental conditions.In addition, console 16 is preferably air conditioned to permitoperation in extreme industrial environments.

Probe 12 is configured to illuminate and collect light scattered fromsolid samples 20 such as are shown in FIG. 2, including layered samples.Probe 12 includes a housing in the form of a generally cylindricalmember 22 and includes a nose cone 24 containing an optical window 26.The optical window 26 can comprise a simple opening through which lightcan pass. In one preferred embodiment the optical window 26 comprises asapphire window mounted within the aperture to protect the optics withinprobe 12 from airborne dust and assorted particles. The probe 12 can beeasily positioned relative to sample 20 by means of handle 28, or can beadapted for robotic manipulation. A trigger 30 is situated on the handle28 for easy operation by an operator's index finger. Alternatively, thetriggering function can be computer controlled. A longitudinal rail 32,shown in cross-section in FIG. 3, is fixed to handle 28, or theequivalent robotic coupling structure, and provides a foundation for theoptical components within the probe 12. The generally cylindricalhousing member 22 includes a longitudinal slot, the edges of whichcontact opposing edges of the longitudinal rail 32. The housing 22 iscompleted by back wall 34. In the preferred embodiment, the generallycylindrical housing member 22 has an internal diameter of about 5.7 cm.It is understood, however, that the internal diameter and otherdimensions of housing member 22 can vary in accordance with theconstraints imposed on the system by its intended use as well as thecomponents to be housed therein. In the preferred embodiment, thehousing member 22, nose cone 24, longitudinal rail 32, and back wall 34are construction of aluminum that has been black anodized. A variety ofmetals, copolymers, and composites may be used, however, to constructprobe 12 in accordance with the present invention.

The longitudinal rail 32 includes a lower surface 31, an upper surface33, a rearward end 35, and a forward end 37 as shown in FIGS. 4 and 5. Aplurality of lateral slots 39 a through 39 g are milled into the uppersurface 33 of the longitudinal rail 32 generally perpendicular to thelength dimension of the longitudinal rail 32, except slot 39 c which isinclined at an angle of about 10°. Pivot pins 38 are fixed in the centerof each of the lateral slots 39 a, 39 c, and 39 e to permit smalladjustments in the alignment of the supports fastened therein. Support46 is fastened in slot 39 f to hold lens 36 adjacent the exit end 41 ofoptical fiber 66 carrying light from the laser source housed in theconsole 16. Support 49 is fastened in slot 39 b to hold a band passfilter 48 which controls the wavelength deviation of the source lightdirected toward the sample 20. Support 51 is fastened in slot 39 a tohold an objective lens system 54 and mirror 50. Supports 46 and 49 alsosupport the ends of baffling tube 47 creating a specific segregatedregion within the housing 22 between the lens 36 and band pass filter48.

Support 52 is fastened in inclined slot 39 c to hold optical filter 76,such as an interference or holographic filter and preferably a long passfilter designed to reflect light having a wavelength equal to or lessthan the wavelength of the laser source and transmit light having awavelength longer than the laser source. Support 56 is fastened in slot39 d to hold a lens 74 having a focal length selected to direct theRaman signal passing through the optical filter 76 on the entrance end53 of spatial filter 55. Support 58 is fastened in slot 39 e to hold theentrance end 53 of spatial filter 55. Support 40 is fastened in slot 39g to hold the exit end 57 of spatial filter 55 that also holds theentrance end of optical fiber bundle 62 that carries the characteristicRaman signal produced from a sample through the fiber-optic umbilical 14to the instruments within console 16.

Essentially monochromatic light from a laser source follows a firstoptical path 78 defined in part by the transmission optics 44 withinprobe 12, shown in FIG. 5. Any light from the source transmitted byfiber end 41 that is rejected by band pass filter 48 is absorbed by theinterior of the baffling tube 47. This is accomplished by constructingthe baffling tube 47 to have a light absorbent interior surface, andinclining the band pass filter 48 with respect to the axis 72 by a smallangle, typically about 3°. The small inclination of band pass filter 48also ensures that any light reflected back from the sample past mirror50 is extinguished on the interior of the baffling tube 47, and is notdirected toward end 41 of optical fiber 66, thereby reducing or avoidingmode hopping of the source light. In the preferred embodiment, thebaffling tube consists essentially of an aluminum tube that has beenanodized satin black to maximize light absorbency.

Light of the correct source wavelength starts at the exit end 41 ofoptical fiber 66 in support 46, passes through band pass filter 48 insupport 49, and reflects off of mirror 50 in support 51toward long passfilter 76 in support 52. The reflective character of the long passfilter 76 for light of the wavelength of the source laser insures thatthe source light will be directed to the objective lens system 54 and onthrough the optical window 26 to sample 20. The inclination of mirror 50is fixed in support 51 while the inclination of supports 51 and 52 canbe adjusted about pivot pins 38 to insure the desired alignment of theoptical path 78. The monochromatic light is scattered by the sample 20creating a Raman signal that is typically about 1 part in 10⁶ of thereflected and scattered incident light. The Raman spectrum appears aslight that is shifted to longer wavelengths from the source laser. Theobserved wavelength shifts are produced by molecular vibrationalfundamentals of various materials. This is graphically presented by FIG.10a where the reflected signal at about the source wavelength is shownas a large signal in the vicinity of the position 0. The Raman signal ishidden within the essentially flat portion of the line extending to theright from the origin representing longer wavelengths.

The portion of the Raman signal which is collected through opticalwindow 26 is collimated by the objective lens system 54 and directedparallel to optical axis 80 back toward fiber bundle 62 as shown inFIGS. 5 and 6. Probe 12 employs sampling optics 42 to collect thescattered Raman radiation, discriminating with an extinction ratio ofabout 10⁶ (1 ppm) or better for the Raman-shifted component. The longpass filter 76 rejects substantially all of the source laser lightreflected from the sample as well as substantially all of the shorterwavelength light incoming from ambient light sources. Still, someun-rejected light at about the wavelength of the source laser is able topass through filter 76. Imaging of the collimated beam from the sampleinto the entrance end 53 of spatial filter 55 is accomplished byfocusing lens 74 in support 56. The signal entering the entrance end 53is illustrated by FIGS. 10b and 10 c (note the change of scale betweenFIGS. 10b and 10 c). The un-rejected light at about the wavelength ofthe source laser coming through filter 76 can swamp some details of theRaman signal. The spatial filter 55 acts to further extinguish theun-rejected light at about the wavelength of the source laser so thatadditional structure of the Raman signal can be detected.

The spatial filter 55 includes a housing 59 creating anotherspecifically segregated region within housing 22 between holder 58 andholder 40. The interior of spatial filter 55 is shown generally in FIG.5, and in more detail in FIG. 6. The spatial filter housing 59 ispreferably construction of aluminum that has been black anodized,however, a variety of other metals, copolymers, and composites may beused to construct the spatial filter housing so that the interior wallwill extinguish any stray light entering through the entrance end 53.The entrance end 53 of the spatial filter 55 includes an aperture 65that is generally round and preferably has an area of about 1 mm² orless. A first lens 67 is positioned to focus on the aperture 65 so thata the radiation passing through the aperture 67 is again collimatedwithin spatial filter housing 59. A holographic notch filter 69 ispositioned to intercept the collimated radiation. The holographic notchfilter 69 is selected to extinguish any remaining radiation of awavelength that would pass through band pass filter 48, thus eliminatingany radiation from the laser source emitted by fiber end 41. A secondlens 71 collects the radiation passing through the notch filter 69 onentrance end 61of optical fiber bundle 62. The Raman signal havingpassed through the spatial filter 55 is significantly more detailed asshown by FIG. 10d.

Imaging of the Raman radiation onto the fiber bundle 62 carried byumbilical 14 can provide, when desired, a depth of field of as much asabout 1 cm, which eliminates the need for precise alignment between thesample 20 and the probe 12 where surface sampling is desired. Where itis desired to sample one or more layers beyond the surface, much moreprecise positioning is required as is a very narrow depth of field. Thedepth of field can be modified by selection of an appropriate objectivelens system 54. The depths of field for various objective lens systemsis shown graphically in FIGS. 14, 15, and 16. It is to be noted that a2.6 mm FWHM depth of field is achieved using a standard 35 mm lens. Evengreater depths of field can be achieved using standard 70 mm or even 105mm lenses. For testing discrete layers of a layered sample, a verynarrow depth of field is desirable such as are shown in FIG. 15 where adepth of field of less than 0.4 mm FWHM is achieved through the use of alens system having at least 20×magnification.

A narrow depth of field can help to retrieve a desired Raman signal froman environment where other Raman signals may be much stronger. Anexample of such an environment is illustrated by the graph in FIG. 12awhere it can be seen that the signal from a layer of PVB plasticsituated between two layers of glass forming a windshield is ordinarilymuch smaller that the glass signal. The details of a PVB signal takenfrom a polymeric sample by itself are shown, at a different scale, inFIG. 12b. The second-derivative vectors of the glass and PVB signals areshown in FIG. 12c. By employing an objective lens system using a50×power, 4mm focal length system having a depth of field of about 0.1mm FWHM, the system is able to extract a PVB signal from a layerwindshield as shown in the lower curve of FIG. 13a. It can be seen thatthis lower curve of FIG. 13a does not appear to reproduce a PVB signaltaken directly from a polymeric sample by itself as shown in the topcurve of FIG. 13a. However, when one considers the second-derivativevectors of the two curves in FIG. 13a, which are shown in FIG. 13b, theyappear very similar.

The probe 12 is coupled to instrument console 16 by fiber-opticumbilical 14. Umbilical 14 includes the mapped optical fiber 62 thatdelivers the scattered Raman radiation from probe 12 to a spectrographlocated in the console 16. Fiber bundle 62 preferably includes nineteenfibers arranged in a close-packed configuration of one fiber surroundedby six fibers surrounded by twelve fibers as shown in detail in FIG. 11.Each of the fibers includes an entrance end 61 positioned to lie withinprobe 12 in a common plane, an exit end 63 positioned to lie withinconsole 16, and a middle portion extending there between. The umbilical14 enters probe 12 through an opening formed in back plate 34 of housing22. The umbilical 14 also includes the optical fiber 66 that carries thelight emitted by the source laser located in the console 16. Theumbilical 14 also carries a communication link between trigger 30 andthe console 16, which generally takes the form of a two wire cable 70that connects trigger 30 is series with an electrical control circuit inconsole 16.

A coherent light source 64 suitable for use with the present inventionis a near-infrared (NIR) Raman excitation laser illustratedschematically in FIG. 7. NIR lasers are preferred as they minimize theabsorption and fluorescence of plastics, pigments, and surfaceimpurities. The use of a multi-mode diode laser, the deviation of whichis compatible with spectra-chemical identification of plastics andcomposites, provides the user with lower cost and higher reliabilitythan a single-mode diode laser. In the preferred embodiment, laser 64 isa 1.2 Watt, 798 nm pulsed multi-mode laser excitation source. Laser 64produces an identifiable signal from almost all materials with afraction of a second exposure. It is understood, however, that laser 64may also be of higher power so long as the pulses are shortened so thatbetween about 0.05 and 2.0 joules of photon energy are delivered to thesample. The laser 64 is actuated by a triggering circuit 68 coupled totrigger 30 and in conjunction with computer 60.

The triggering circuit 68 initiates the operation of the laser 64 whichoperates in a pulse mode between a lower and an upper intensity. Thelower intensity is a rest mode just at or slightly above the thresholdbelow which laser activity is discontinues in the laser. The upperintensity can be adjusted to any level at or below a maximum intensityestablished by the upper power limit of the laser. The triggeringcircuit 68 in conjunction with the computer 60 can define the shape andduration of the pulse as well as a minimum “lapse time” or “off time”between the laser pulses to allow the sample 20 to dissipate localizedheat generated by the laser pulse. Advantageously, a single location ona sample 20 can be exposed to more than one pulse during the samplingprocess, and allowed to cool between pulses. The laser pulses are cycledrepeatedly either in response to a continuous depression of the trigger30, or as quickly as the trigger 30 is depressed, but not more oftenthan permitted by the lapse time setting, until an adequate Raman signalis collected to allow the required analysis to identify the sample. Thepulse width defining the duration of the upper intensity can be betweenabout 0.01 and 20.0 seconds. Typically, the number of pulses employed totest a given sample can be in the range of from 1 to about 30 pluses. Bycontrolling the pulse duration and lapse time, the triggering circuitdefines a range of power densities that can be provided at the samplesurface which maximizes the Raman signal yet minimizes the thermalstrain on the sample. Thus, the system 10 generally acquires ahigh-quality Raman spectrum of a sample or scrap of most materials witha typical exposure time of less than 500 milliseconds, typically between80 and 120 milliseconds in the single pulse mode.

A system 10 in accordance with the present invention can collectscattered laser light from a sample containing a characteristic Ramansignal over a relatively large focal plane by use of around-entrance-aperture multi-element fiber bundle 62. Collection of theRaman signal is optimized by using a fiber bundle having a diameterequal to that of the magnified image of the area over which the sampleis irradiated. This arrangement maximizes the depth of field for Ramancollection at a given resolution (e.g. about 1 cm depth of field for 40cm⁻¹ resolution) and minimizes the requirement for precise samplealignment. The laser spectral width and collection fiber diameter (whichdetermines the effective reentrance slit-width) are chosen to provide aresolution that matches the intrinsic widths of the Raman signalspectral features. The exit ends 63 of the fibers in bundle 62 arepositioned to lie in a single line stacked to match the dimensions of anentrance end 82 of a spectrograph 84 located in console 16, showndiagrammatically in FIGS. 7 and 9. A spectrograph 84 suitable for usewith the present invention is preferably a 150 mm folded beamspectrometer. The single line defined by the exit ends 63 of fiberbundle 62 is parallel to the ruled lines on grating 83 within thespectrograph 84. The ruled grating 83 causes the Raman signal (and otheraccompanying light) to be dispersed into its spectral components ontothe optical detector array 86. A suitable detector array for use in thepresent invention is a Princeton Instruments model RTE/CCD-256spectrometric detector. The detector 86 is controlled by computer 60 inconjunction with trigger circuit 68 so that data collection is initiatedas the laser source pulse begins to rise from the lower thresholdintensity to the specified upper intensity. The initiation of datacollection can be achieved by opening a shutter to the CCD arraydetector 86. The detector 86 collects the Raman spectrum and stores theresulting digital record as an image during a period of time specifiedby CCD controller 87. The image is subsequently read out to computer 90for further processing in accordance with the present invention. Thecomputer 90 usable in the present invention is an IBM 233 Mhz 128megabyte computer handled through CCD controller 87. The typical commandto recognition time is about 1 second.

Computer 90 employs a background subtraction routine that uses an entirebaseline determination that is adaptive, in which peaks are activelysearched for and the base is established between the peaks over the fullspectrum. In addition, computer 90 processes the Raman signal image readfrom the array detector 86 with a second-derivative vector with highdimensional multi-variant projection. The vector is essentiallyn-dimensional where n is the number of pixels in the detector array 86.The computer 90 uses image processing routines to reduce the collectedspectra to mathematical objects that resemble universal bar codes forcomparison with mathematical objects scored in the library. Theidentification algorithm generates a numerical index between 1 and 0.Any value above the user-supplied threshold results in a positiveidentification which is displayed to the monitor screen. A value belowthreshold, or two values within a specified interval of each otherproduces a result of no identification as generally depicted in the flowchart in FIG. 8.

As a starting point, the computer 90 is loaded with a library of imagestaken from materials of known composition. The library can beuser-extended to an arbitrary number of different materials. The basisfor identification of a sample the Raman spectrum which, as illustratedin FIGS. 11a through 11 d, are quite distinctive for differentmaterials. As indicated previously, the Raman spectrum appears as lightthat is shifted to longer wavelengths from the source laser. Theobserved wavelength shifts are produced by molecular vibrationalfundamentals of various materials. These Raman spectral featurestypically appear as 5-50 peaks each of whose widths are less than orcomparable to the spacing between peaks. As between samples of the samematerial, the spectral features are identical but the intensity of thefeatures may vary depending upon the character of the light sourcecausing the material to produce the Raman spectrum. To overcome thisvariation, the second-derivative vector is computed as shown in FIGS.12c and 13 b. The second-derivative vector of the spectral informationallows a number of mathematical operations to be performed on theinformation taken from the sample.

To load the library, an ambient light reading is taken by the probe andthe image is detected by the detector 86. The image detected by thedetector 86 is then processed by computer 90 to develop thesecond-derivative vector which is stored in the computer. Each of thematerials of known composition is exposed to the probe 20 and thesecond-derivative vector for each known material is computed. The secondderivative vector for the ambient light is then subtracted from thesecond-derivative vector of each of the materials of known compositionand the difference stored with an identification of each known materialin the library. The subtraction can be performed by redefining thelibrary vectors so that they are made orthogonal to the ambient lightvector. The subtraction can also be performed by a masking operationconsisting essentially of identifying portions of the ambient vectorwhere the value is above a selected criterion, and storing a zero valuefor such regions so that only areas outside those regions can beemployed for comparison and identification. Other vector subtractionoperations can be used without departing from the present invention.

The identification of the composition of a material is achieved byperforming the operation outlined in FIG. 8. The trigger 30 on handle 28of the probe 12 is depressed as the probe is positioned less than 1 cmfrom a sample of an unknown composition. This initiates the acquisitionof a Raman signal from the material and its detection by the arraydetector 86. The second-derivative vector is then computed and theambient light vector is subtracted. The resulting sample vector is thencompared to the library of vectors. While a number of mathematicaloperations can be employed for this comparison step, the preferredmethod is to compute the dot product of the sample vector with eachlibrary vector. The dot product yields a scalar number that can beconsidered as a correlation value where 1 is perfect correlation of thetwo vectors and 0 is perfect anti-correlation. While one could merelylook at the entire library for the highest correlation value, such amethod might not always have the reliability one would want. In thepreferred embodiment, an initial collection of identification candidatesis assembled by merely collecting all library vectors having acorrelation with the unknown sample above a user defined value. Thiscollection is then surveyed to identify the top two library vectors andthe difference in correlation values is computed for the top twovectors. If this difference in correlation value is greater than a userdefined level, then the system has successfully discriminated betweenthe two closest candidates, and the identification of the material isnow known. If the difference in correlation values is very small then nounique material identification is possible without gathering additionaldata. Using the present invention plastics are now identified withtypical correlations of about 0.95 or better against 0.20 or less formisses.

Although the chemical information content in a Raman spectrum is similarto that in a NIR spectrum, Raman scattering is superior to NIR in havinga greater penetration depth (about 100 m to 1 cm for Raman as opposed to1 m to 10 m for NIR). This larger penetration depth makes Raman samplingless sensitive to surface roughness and contamination than NIR.Furthermore, the fiber-bundle signal collection system makes the device10 far less sensitive to precise placement of the plastic sample,depending on the depth of field employed.

Raman spectral features are significantly sharper and better separatedthan NIR spectral features. Thus the chemical information content of aRaman spectrum is higher than a NIR spectrum. This higher informationcontent not only allows identification to be performed with a lowererror rate, but in addition makes it possible to qualitatively determinethe chemical composition of copolymers and composites with a higherdegree of accuracy than NIR.

Although the invention has been described in detail with reference to apreferred embodiment, variations and modifications exist within thescope and spirit of the invention as described and defined in thefollowing claims.

What is claimed is:
 1. Optical apparatus for use in an instrument foridentifying the composition of samples from characteristic Raman spectracomprising: a light source of known wavelength and deviation,transmission optics coupled to the light source so that light emittedfrom the light source is directed toward a sample causing the sample toproduce a characteristic Raman spectrum, sampling optics situated toreceive the characteristic Raman spectrum produced from the sample, thesampling optics including: an objective lens system situated adjacent tothe sample to receive radiation reflected from and emitted by the sampleincluding the characteristic Raman spectrum produced by the sample andgenerate a collimated beam of the radiation, an optical filterpositioned to receive the collimated beam of radiation from theobjective lens and allow past primarily only a selected spectral portionincluding the characteristic Raman spectrum produced from the sample, alens positioned to received the selected spectral portion passingthrough the optical filter and direct it toward a focal point, a spatialfilter having an input positioned at about the focal point to receivethe selected spectral portion and having an output, the spatial filterattenuating unwanted radiation so that the output receives substantiallyonly the characteristic Raman spectrum produced from the sample, and anoptical fiber bundle coupled to the output of the spatial filter fortransmitting the characteristic Raman spectrum produced from the sampleto apparatus for identifying the composition of the sample from thecharacteristic Raman spectra.
 2. The optical apparatus of claim 1wherein the spatial filter comprises an enclosure having an inputwindow, a first lens aligned with the input window for collimating thespectral portion passing through the input window, a wavelength filtersituated to filter the collimated spectral portion passing through thefirst lens, and a second lens for directing the spectral portion passingthrough the wavelength filter toward the output.
 3. The opticalapparatus of claim 2 wherein the input window of the spatial filtercomprises an area of about 1 mm².
 4. The optical apparatus of claim 2wherein the wavelength filter situated within the spatial filtercomprises a holographic notch filter selected to reject radiation of awavelength corresponding to the light source wavelength.
 5. The opticalapparatus of claim 1 wherein the objective lens system has a depth offield of less than 1 mm (FWHM).
 6. The optical apparatus of claim 5wherein the objective lens system has a depth of field of less than 0.2mm (FWHM).
 7. The optical apparatus of claim 1 wherein the objectivelens system has a focal length of less than 10 mm.
 8. The opticalapparatus of claim 7 wherein the objective lens system has a focallength of less than 5 mm.
 9. The optical apparatus of claim 1 whereinthe objective lens system has a magnification power of at least 20×. 10.The optical apparatus of claim 9 wherein the objective lens system has amagnification power of at least 40×.
 11. The optical apparatus of claim1 wherein the objective lens system has a working distance sufficient topermit the sampling optics to receive radiation from beyond an adjacentsurface of the sample.
 12. The optical apparatus of claim 1 wherein theoptical filter positioned to receive the collimated beam of radiationfrom the sample is also positioned to reflect the light emitted from thelight source toward the sample.
 13. The optical apparatus of claim 1wherein the optical filter positioned to receive the collimated beam ofradiation from the sample comprises a long pass filter only allowingtransmission of radiation having a wavelength longer than the sourcewavelength.
 14. Optical apparatus for use in an instrument foridentifying the composition of samples from characteristic Raman spectracomprising: a light source of known wavelength and deviation,transmission optics coupled to the light source so that light emittedfrom the light source is directed toward a sample causing the sample toproduce a characteristic Raman spectrum, sampling optics situated toreceive the characteristic Raman spectrum produced from the sample, thesampling optics including: an objective lens system situated adjacent tothe sample to receive radiation reflected from and emitted by the sampleincluding the characteristic Raman spectrum produced by the sample andgenerate a collimated beam of the radiation, the objective lens systemhaving a depth of field of less than 1 mm (FWHM), a focal length of lessthan 10 mm, and a magnification power of at least 20×, an optical filterpositioned to receive the collimated beam of radiation from theobjective lens and allow past primarily only a selected spectral portionincluding the characteristic Raman spectrum produced from the sample, alens positioned to received the selected spectral portion passingthrough the optical filter and direct it toward a focal point, a spatialfilter having an input positioned at the focal point to receive theselected spectral portion and having an output, the spatial filterattenuating unwanted radiation so that the output receives substantiallyonly the characteristic Raman spectrum produced from the sample, and anoptical fiber bundle coupled to the output of the spatial filter fortransmitting the characteristic Raman spectrum produced from the sampleto apparatus for identifying the composition of the sample from thecharacteristic Raman spectra.
 15. The optical apparatus of claim 14wherein the spatial filter comprises an enclosure having an inputwindow, a first lens aligned with the input window for collimating thespectral portion passing through the input window, a holographic notchfilter selected to reject radiation of a wavelength corresponding to thelight source wavelength filter and situated to filter the collimatedspectral portion passing through the first lens, and a second lens fordirecting the spectral portion passing through the holographic notchfilter toward the output.
 16. The optical apparatus of claim 15 whereinthe input window of the spatial filter comprises an area of about 1 mm².17. The optical apparatus of claim 14 wherein the objective lens systemhas a depth of field of less than 0.2 mm (FWHM), a focal length of lessthan 5 mm, and a magnification power of at least 40×.
 18. The opticalapparatus of claim 14 wherein the objective lens system has a workingdistance sufficient to permit the sampling optics to receive radiationfrom beyond an adjacent surface of the sample.
 19. The optical apparatusof claim 14 wherein the optical filter positioned to receive thecollimated beam of radiation from the sample is also positioned toreflect the light emitted from the light source toward the sample. 20.The optical apparatus of claim 19 wherein the optical filter positionedto receive the collimated beam of radiation from the sample comprises along pass filter only allowing transmission of radiation having awavelength longer than the source wavelength.